This invention relates generally to fuel-air optimization in cannular gas turbine combustors and more particularly concerns a system for continual on-line trimming of the fuel flow rate to each can of a cannular combustor.
FIG. 1A shows a conventional gas turbine apparatus which includes a compressor 2, a combustor 3 and a turbine 4. Fuel is mixed with compressed air from the compressor 2 and burned in the combustor 3. The resulting flow of combustion products out of the combustor 3 drives the turbine 4 which in turn drives a generator 5 as well as the compressor 2. The exhaust from the turbine 4 is eventually released to the atmosphere. One type of combustor commonly used today is the so-called cannular combustor which comprises a plurality of separate cans, wherein each can is connected to the compressor 2 and the fuel supply and outputs to the turbine 4.
FIG. 1B schematically shows a single can 10 of a conventional cannular combustor. The can 10 defines a tubular combustion chamber 11 to which a fuel-air mixture from a premixer 12 is injected. Air at compressor discharge conditions enters the premixer 12 via an air line 13 and fuel enters via a fuel line 14. A main valve 15 is disposed in the fuel line 14 to throttle the flow of fuel into the premixer 12. Alternatively, the fuel and air may be directly injected into the combustion chamber 11 without premixing. This results in near-stoichiometric, high temperature combustion which leads to copious production of NO and NO.sub.2 which are generally referred to as NO.sub.x. Premixing the fuel and air prior to combustion results in lean premixed combustion which produces lower flame temperatures and thus lower NO.sub.x emissions.
Reducing emissions of harmful gases such as NO.sub.x into the atmosphere is of prime concern. Gas turbine-based power plants burning natural gas offer a means for dramatically reducing these emissions. Natural gas-fired gas turbines produce no measurable particulates or SO.sub.x and, if the combustion process is properly controlled, very little NO.sub.x or CO. NO.sub.x emissions are very sensitive to fuel-air ratio, increasing exponentially with respect to the fuel-air ratio. Because of the non-linear increase, total NO.sub.x emissions for a prescribed amount of total fuel can be minimized when the fuel-air ratio is the same in all cans of a cannular combustor. However, uniformity of air flow to each can cannot be controlled to better than 3-4% and the can-to-can variation is not known in real time. These uncontrollable can-to-can variations in air flow mean that unless individual can fuel inputs can be accurately controlled, the fuel-air ratios between cans will not be uniform and excess NO.sub.x will be produced.
Accordingly, there is a need for real time, on-line trimming of the fuel flow to each can of a cannular combustor in accordance with minimizing total NO.sub.x emissions. There is an additional need for a trim system to carry out the real time, on-line trimming which is retrofittable to existing cannular gas turbines. The trim system must be such that its failure will not affect the baseline operation of the gas turbine apparatus.